The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to equipment and methods for completing subterranean wells; in particular, wells in which large temperature fluctuations may occur.
During completion of a subterranean well, drilling and cementing operations are performed to provide a conduit through which desirable fluids originating within the formation may flow. The cementing operation involves placing a competent cement sheath inside the annular region between the external surface of a tubular body such as well casing, and the borehole wall. The cement sheath supports the casing and provides a hydraulic seal between producing formations. The presence of a hydraulic seal is commonly called zonal isolation.
Well cementing is a difficult operation because it requires several parameters to be considered and controlled. Such parameters include density, setting time, rheological properties, fluid-loss rate, set-cement strength and permeability. Control of these parameters is inherent to any well-cementing operation, and is well known to the skilled person. Solutions generally involve incorporating various additives into the cement slurry. Detailed information about well cementing may be found in the following publication: Nelson E B and Guillot D (eds.): Well Cementing, 2nd Edition, Schlumberger (2006).
When well cementing is successful, and a cement sheath has been formed that provides casing support and zonal isolation once the slurry has set and hardened, it may not be long before the sheath is subjected to mechanical and/or thermal stresses that can lead to deterioration. Cement systems employed in thermal-recovery wells are particularly prone to problems that lead to loss of zonal isolation. One type of thermal-recovery well involves the injection of steam into the wellbore, commonly known as steamflooding. Steamflooding may consist of introducing steam into an injection well and sending the steam through the formation to one or more production wells. Another technique involves cyclic steam injection, during which steam is injected into a single well for a limited period. After the steam-injection period, the well is placed into production. Heating the reservoir reduces the viscosity of oil in the formation, making production more efficient. Steamflood wells are usually less than 915 m (3000 ft) deep, and are frequently deviated (30° to horizontal). The circulating temperatures during primary-cementing operations are often less than 40° C. (104° F.). During injection, the steam temperature usually approaches about 315° C. (600° F.).
When heat is initially supplied, the temperature rise is normally controlled to prevent undue thermal shock to the casing and cement. Nevertheless, because of thermal expansion, high levels of stress are built up in the pipe and the cement sheath.
A substantial amount of work has been performed for many years to devise cementing techniques that minimize the effects of thermal stresses. Such methods include the placement of thermal packers and the inclusion of a sliding sleeve in the casing string that can move freely in response to thermal stress. Another procedure involves holding the casing in tension during primary cementing to minimize axial casing deformation when thermal stress is eventually applied.
More recently, a method was developed that involves applying internal casing pressure after the primary cementing has been performed, and while the cement slurry is setting and hardening. The internal pressure may vary from about 15.9 MPa to 138 MPa (2300 psi to 20,000 psi). This process prestresses the casing, and gives the cement sheath an improved ability to withstand the application of heat during the steam-injection process. Modeling software is used to analyze the anticipated well conditions during steam production, and determine the optimal amount of casing pressurization. All of the above techniques are aimed at maintaining zonal isolation.
There are also situations during which the casing and cement may experience a temperature decrease. For example, cold fluids may be injected into subterranean formations during hydraulic fracturing operations, or to maintain reservoir pressure. In other cases involving long casing strings or long liners, the cement at the top may initially be hotter as it sets than the surrounding formation. In addition to the heat of cement hydration, the slurry would have been further heated during its journey along the bottom of the string. As the casing-cement-formation system returns to equilibrium, the temperature decreases. In addition, casing and cement cooling may occur in shallow sections of deepwater wells. Such cooling may also induce stresses and deformation in the casing and cement sheath that could lead to the loss of zonal isolation.